1. Field of the Invention
The present invention relates to a set point change-over circuit for analog-type or digital-type fluid control valves and more particularly to such set point change-over circuit so designed that the application of electric signal set point to a fluid control valve is changed over to one of a plurality of set points predetermined as fluid pressure or flow controlling references selected in accordance with the set point select instruction from an external control circuit such as a sequencer.
2. Description of the Prior Art
To effect the change-over between the set points of a fluid control valve of the type in which the fluid pressure or flow rate is controlled analogically or digitally in accordance with the magnitude of an electric input set point signal, it has been the usual practice in the past such that the change-over is effected by selecting desired one of a plurality of set points preliminarily set in setting means by a sequential controller. The sequential controller usually comprises a microcomputer, relays or the like and it is disadvantageous in that during the change-over from one set point to another the two set points are simultaneously applied to the control valve over a period of 1 to 2 ms or conversely there is a moment when neither of the set points is applied thus causing a non-input state. Thus, there are disadvantages that during the change-over period an unstable voltage is generated thus causing the fluid pressure or fluid flow rate to undergo a shock through the fluid control valve, that the absorption of such shock requires the provision of a waiting time or the use of the control valve with a degraded response characteristic through the provision of a first order lag circuit or the like and so on.